[Haskell-cafe] Simple but interesting (for me) problem

Gregory Crosswhite gcross at phys.washington.edu
Thu Oct 22 11:43:52 EDT 2009

Thank you for the additional explanation, but it isn't clear that what  
you have added is inconsistent with my explanation.

The point I was trying to make is that in an impure/imperative world,  
you may assume that a function is called every time that you use it.   
However, in a pure world the assumption is that a function called with  
the same arguments will always return the same result (i.e.,  
referential transparency) so that you only need to run it's code once  
and then you can re-use that value henceforth.

In practice, of course, what happens under the hood (at least, with  
GHC) is that "foo <- mkNext" constructs a thunk named "foo" which is  
evaluated the at the first "print foo" and from then on the thunk is  
in an evaluated state and so later references to it just use this  
value rather than re-evaluating it.  This is because, due to  
referential transparency, it is equivalent to think of foo both as a  
function whose value can be cached and as a constant value that we  
just don't know yet.

The problem with the "foo" that was defined is that its code will  
actually give you a different value each time that you run it,  
violating the semantics of the language since all functions are  
assumed to be pure.  The problem with violating this semantic is that  
the compiler uses it whenever it can to make things more efficient,  
which in this case means treating foo as a value that only needs to be  
evaluated once even though each time you run the code you actually get  
a different result.  Hence, the results are in a sense undefined since  
the compiler is allowed to run foo as many times as it wants expecting  
to get the same result each time;  for example if two threads  
evaluated foo at the same time then under pathological conditions the  
first thread might see "1" and the second thread "2".

So the moral of this story --- and perhaps the point that you were  
trying to make --- is that it is better to think of "foo" as a  
constant value that you just don't know yet (until you evaluate) it  
rather than as a function that you can call.

(Your nitpick that "next" would have been a better name than "foo" is  
well taken, though.)


On Oct 22, 2009, at 12:48 AM, minh thu wrote:

> 2009/10/21 Gregory Crosswhite <gcross at phys.washington.edu>:
>> And just because this has not been explicitly stated:  it's not  
>> just for
>> aesthetic reasons that you couldn't do this with a pure function, but
>> because it violates the semantics and gets you the wrong result.   
>> So for
>> example, if you modified Tim's code to be
>> import Data.IORef
>> import System.IO.Unsafe
>> mkNext :: (Num a) => IO a
>> mkNext = do
>>  ref <- newIORef 0
>>  return . unsafePerformIO $
>>         do
>>           modifyIORef ref (+1)
>>           readIORef ref
>> main :: IO ()
>> main = do
>>  foo <- mkNext
>>  print foo
>>  print foo
>>  print foo
>> Then the output that you will see (with GHC at least) is
>> 1
>> 1
>> 1
>> because the compiler assumes that it only needs to evaluate foo  
>> once, after
>> which it can cache the result due to assumed referential  
>> transparency.
>> - Greg
> This is indeed wrong, but not how you think it is.
> The code you pass to unsafePerformIO has type Num a => IO a, so the
> value passed to return has type Num a. So foo has type Num a too and
> its value is 1.
> Exactly like in
> mkNext = do
>  ref <- newIORef 0
>  modifyIORef ref (+1)
>  readIORef ref
> which is a complicated way to write
> mkNext = return 1
> Now, it's clear that foo has value 1 and printing it three times
> should output three 1. The whole point of having mkNext return an
> action (that should be called next, and not foo, as it is much
> clearer) in previous code was too be able to execute it multiple times
> and having it return a new value.
> In general, expecting
> print bar
> print bar
> print bar
> outputing three different things is wrong, as bar should be pure. If
> bar is not pure, then it should be
> a <- bar
> print a
> b <- bar
> print b
> c <- bar
> print c
> Cheers,
> Thu

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